Producing accurate and useful crime maps is far harder than it appears and does not rely only on geographical information, the Home Office has been warned, as the deadline approaches for getting every force in England and Wales to produce them.

With only a couple of working weeks left before the end of the month - by which time all the relevant forces should have crime maps, according to the plan set out in July by Jacqui Smith, the home secretary - Pitney Bowes MapInfo has released a white paper on best practice.

It says problems will arise because a significant amount of crime goes unrecorded, location may be uncertain, and time of day, seasons and even the activities of the police will make figures vary.

The white paper suggests a wide amount of consultation is required, with local authorities, social, health and emergency services, MPs, community groups, "crime pattern influencers" (such as bars and pubs), business groups and "Freedom of Information enquiry concentrations" - presumably, concerned residents.

Averaging should be avoided, and point data used, it says - which is the reverse of the approach being adopted by a number of police forces, including the Metropolitan Police. Maps should also have overlays to explain crime spikes and have day/night splits.

The Free Our Data campaign thinks the practices outlined in the memo do not go far enough: what external developers especially are looking for is pure data feeds of events, rather than static maps. Such a data feed, with location and time data, enabled the creation of the first useful crime mashup, at Chicagocrime.org.

Ironically, the police's efforts to meet the deadlines might be better aimed at producing those data feeds with time, location and crime form data which could then be used by external developers - who would be able to produce maps more quickly than in-house efforts.